The present invention generally relates to battery modules and in particular to connection and control of multiple battery modules.
Batteries and battery packs are used as power sources for many different applications. However, because of the variety of different applications that batteries can be used, each particular application typically has its own operating parameters including a voltage requirement. As a result, either the parameters associated with the battery must be designed to meet the particular application or individual batteries must be strung together to provide the required output voltage, for example. The first method is inefficient because the battery is limited in the number of applications in which it may be used. On the other hand, the second method has the problem associated with the potential for large offset voltages when a number of cells are connected together to form, for example, a high voltage battery.
High voltage batteries have many useful applications. For example, high voltage batteries may be used in vehicles as an alternative to conventional power sources. In order to provide a high voltage battery necessary for such an application, multiple batteries are placed in series to provide the desired output voltage. High voltage batteries may also be subjected to a wide range of different operating conditions. For example, battery operating conditions may change due to temperature, load, charging, age, etc, throughout the operational life of the battery. In order to ensure that the battery continues to operate at an optimal level, and to prevent damage to the battery or the area containing the batteries, it is also desirable to monitor the batteries and their individual cells. In particular, the batteries may be monitored for, among other things, over discharge conditions, under voltage conditions, and temperature variations.
Monitoring of high voltage batteries is difficult, however, because of the large offset voltages that occur from stringing multiple batteries together in series. In order to determine the local voltage level at any particular stage in the string of batteries the local voltage level must be determined with reference to that battery""s high voltage and ground terminals. However, the ground of any individual battery fluctuates relative to the preceding battery""s xe2x80x9chighxe2x80x9d voltage. Furthermore, any difference is compounded the further removed that any particular battery is removed from true ground. As a result, determining a battery""s offset is not a trivial matter.
Monitoring devices that are connected to the cells of a high voltage battery must overcome the increased offset voltages as the monitoring device is connected to cells that are further from true ground. Conventionally, the large offset associated with high voltage batteries is overcome by providing galvanic isolation. Typical galvanic isolation techniques include the use of optical isolators, capacitors, or transformers. According to the use of these devices, the bandwidth of the isolation device must be carefully selected to match the bandwidth of information, based on the frequency of the information, that is being communicated to the monitoring equipment.
If the information being communicated to the monitoring equipment is in the DC or sub-hertz bandwidth, a telemetry system is often used to convert the data to a higher frequency band via analog to digital converters or through voltage-to-frequency/frequency-to-voltage devices. The converted higher frequency information is then transmitted across a galvanic isolation barrier to be interpreted as the information that is being monitored. The information that is to be monitored can be supplied through optical isolators as digital information. Alternatively, the information may also be provided through capacitor coupling or pulse transformers.
However, systems using galvanic isolation to overcome the large offsets associated with high voltage batteries have drawbacks in terms of both performance and cost. For example, although optical isolators have very high galvanic isolation performance, they are slow and very expensive. On the other hand, AC coupling provides a relatively low cost solution that grows in an amount inversely proportional to the frequency of the monitoring signal. In addition, AC coupling also causes large amounts of distortion to squarewave signals. For example, low frequency signals require very large capacitors. Transformers also become increasingly expensive and heavy as the frequency of the monitoring is lowered.
In addition, there are also important considerations with regard to the telemetry signal format that have to be taken into account when choosing the type of galvanic isolation that is to be used. For example, the waveform generated by Manchester encoding is easily accommodated by capacitor or transformer coupling since it is regular and continuous. Such a waveform contains a synchronized clock encoded in its content and is pulsewidth modulated. On the other hand, a Non-Return-to Zero (NRZ) data stream, sent through a serial communications interface to or from a PC RS-232 cable, for example, is not continuous and contains large DC components that are not transferable through AC or transformer coupling. As a result, the standard approach uses optical isolation for the type of telemetry needed for monitoring of battery modules.
Conventionally, if multiple batteries are strung together in series to provide a high voltage battery, any monitoring processor is connected directly, via optical couplers to each individual battery. As a result, this arrangement requires a large number of inputs to the host processor performing the monitoring operations. Furthermore, the host processor also has to maintain and manage a large number of separate communication links. This results in a large overhead that is added to the processor""s responsibilities in addition to the expense associated with using optical isolators.
It is therefore an object of the invention to control battery output in finite unit increments, while obviating the need for expensive components such as optical couplers.
It is another object of the invention to provide a simple and effective monitoring of a battery while reducing the communications overhead associated with the telemetry and monitoring performed by a host processor controlling the battery.
It is a further object of the invention to provide a redundant modular battery that is easy to monitor and that may be connected in any order to create a desired voltage output.
It is a yet further object of the invention to provide a robust modular battery capable of identifying and signaling battery conditions in addition to communicating and facilitating the identification of battery warnings and alarms.
According to an exemplary embodiment of the present invention, the foregoing and other objects are accomplished through implementation of a serial communication interface including a bus voltage level translator and voltage safety interlock system. According to a preferred embodiment of the invention, a voltage level translation interface (VLTI) is used to send Uplink and Downlink serial commands and data to a string of battery modules from a central monitoring processor. In addition, each module contains a transmitter, receiver, and microcontroller to monitor commands addressed to the module on the interface and provide answers to the central processor. In addition, an end module can initiate a warning signal to the central processor for various battery conditions via a safety interlock system in addition to the downlink communication path. Through implementation of the VLTI design according to the various exemplary embodiments of the present invention, the addition of as many modules as required to form the desired high battery voltage can be accommodated, while using low cost components. Furthermore, no optical couplers or other traditional DC blocking devices such as transformers, capacitors, or special isolation amplifiers are required in order to accommodate the large offset voltages associated with high voltage batteries. According to the invention, this is accomplished by level shifting each bit stream of commands up to the high potential of next module in the uplink data stream. Similarly in the downlink data stream, each bit stream is level shifted from the present potential to the lower potential of the next lower module. The level shifting can be achieved using standard transistor and resistor circuits providing a lower cost alternative to conventional galvanic isolation.